Factors influencing the frequency of 6-thioguaninine resistant mutations were examined in Chinese hamster lung cells. The increment in the plating efficiency of treated cells as a function of expression time, spontaneous mutation rate per cell per generation, viability of mutant as well as wild-type cells, and half life of HGPTase were also evaluated. Continuous diminution in the level of HGPTase was interpreted as an important factor responsible for the recovery of mutants during the expression time. This cell line was suggested to be suitable for mutagenicity testing in the induction of mutation at the HGPRT locus.